For operators of passenger vehicles, it is of utmost importance to minimize maintenance costs and downtime. To this end, passenger vehicle components and subsystems are modularized to facilitate replacement. In aircraft, to enable operators to quickly and easily remove and replace faulty, broken or otherwise malfunctioning parts, many components are installed during assembly as line replaceable units (LRUs). Typically, LRUs are removed and replaced by the operator's maintenance staff (and often at the LRU manufacturer's cost, for example, if the LRU is under warranty) at the first indication of irregular operation regardless of whether the LRU has truly malfunctioned. Often, a normally-operating LRU is replaced unnecessarily because the LRU simply has an appearance or isolated instance of irregular operation, for example due to user error in operating the LRU.
One such aircraft LRU that has been replaced unnecessarily is the combination chiller/refrigerator/freezer unit (hereinafter referred to as a refrigeration unit) that is installed in the aircraft's galley. Conventional refrigeration units are user-settable for a temperature set-point. In some instances, however, aircraft staff (e.g., inexperienced flight attendants) may mis-set the temperature set-point relative to the type of items being stored in the refrigeration unit, thereby causing item spoilage. In yet other instances, aircraft staff may close the door to the refrigeration unit but fail to notice that the door was not properly closed and, therefore, the refrigeration unit may operate inefficiently and not properly cool the items being stored inside. In view of the foregoing, a refrigeration unit including a diagnostic means for discriminating between user error and unit malfunction would be an important improvement in the art.